Echo suppressor



July 14,1931.

D. MITCHELL 1111110 surrnssoa Filed Feb. 2o, 1929 1 INVENTOR Y Jm-C/eaATTORNEY Patented July 14, 1931 l' UNITED STATES PATENTfol-"Ficav DOBENMITCHELL, oF NEW YORK, N. Y., AssIGNoR rro AMERICANTELEPHQNE AND hTELEGRAPH COMPANYLA CORPORATION OF NEW YORK ECHO sUPPREssoR Applicationfiled February 20,1929. seriarivo. 341,543.

'This invention relates to two-way transmission circuits, andmoreparticularly to four-wire circuits and the apparatus often associatedtherewith for suppressing echoes.

For a discussion of some of the fundamental problems connected with thepres-y ence of echoes in two-way transmission cir-V cuits and for adescription of echo suppressing devices which are among those betterXknown inthe art, reference may be had to article by A. B. Clark and R.C. Mathes, Journal of the AmericanInstitute of Electrical Engineers,June', Vl925,.pages 618-626,..

The applicants invention is applicable to u] voice-operated echoSuppressors such as that disclosed in the above identifiedy article. Theprincipal object of the invention is to prevent false operation of theecho suppressor relays by noise n ing the sensitivity of the echosuppressor.

In attaining the above stated object7 the applicant in generalassociates with the de-` tector tubes means responsive to noise in thetransmission circuit for raising the threshold of operation of thedetector. to a degree just sufficient to prevent operation by thenoise,.along with means for producing the necessarylowering fthethreshold when the noise is'decreased. y n The invention will beclearlyuiiderstood A when the following detailed description ofone'desirable embodiment thereof i'is read with reference to theaccompanying drawing, which shows -diagrammatically the circuits of theimproved echo suppressor.

With reference to the details of the drawing, there is shown a sectionof-a four-wire circuit having the upper two-wire Vline L1 adapted fortransmission from west to east, for instance, and the lower two-wireline L2 adapted for transmission fromeast towest. Connected across theline L1 is a vacuum tube amplifier A1 which has its output associatedwith the input of la three-electrode vacuum tube detector`D1- In thegrid-filament circuit of the detector D1 area con# denser C1 which ispreferably Vof several microfarads, and a resistance fr which may have a:value of several hundred thousand n ohms.

without excessive reducings connected in series inthe plate circuit ofdetector D1. Relay R1 controls the circuittothe usual echo-suppressorrelays directly controlling the blocking of line L2 in response to thewaves in line Ll, this appara-l release relay R2. VN ext in order, withrespect' R3 which,` as indica-tedv Y to sensitivity, is relay above,` isslowreleasing and controls a slow operating relayRi, as indicated in thedrawing. Relay Rl', which may be termedthe Three relays'Rh'IR'hRa havetheir windechosuppressorrelay, ismade slightly less sensitive thanvrelay R3. As Awill appear more clearly hereinafter, relay R4 controlsiii .part the charging circuit `of condenser C1.'

lVhileit is quite feasible to utilize relays R2,' R3, and R4 to controlthe sensitivity o f the opposite echo suppressor, as well as the'sensitivity `of the apparatusnow under'dis cussioii, by, suitableconnectionV ofY extra windings o'f relaysR2 and R3,Hsuch an ar-lrangement has one disadvantage, as will appear hereinafter, andaccordingly these relays are duplicated on `the opposite 4side of thecircuit. vConnected across the line Lg'islthe amplifier-.detectorarrangement Ag-DT A condenser C2 and a resist'ance W1 in the input ofthe detector D2 correspond tothev elemeans Cland .r1 on the other sideof the circuit.k I n the plate circuit of thedetector Relays R-2 VrelayRflk slightly less sensitive thanrelay l It isjbelieved thatan..understandiiig of the invention Will'bebestadvanced by the followingdescription of the operation of the circuits.

lf a talker at the west end oi' the circuit produces waves travelingfrom left to right in line L1, it is of course desired to block theopposite transmission path L2 in order to cut off thereturning echoesand to effect certain other switching operations, for a discussion ofwhich reference may be had to the above identied Clark and Mathesarticle.

Futhermore, it is of course desirable that the echo suppressor apparatusshould have considerable sensitivity in order' thatV all appreciableechoes may be suppressed. However, it the echo suppressor has a fixedsensitivity, it will readily be understood that operation may resultfrom noise currents in the transmission path and that false blocking ol'the four-wire circuit will thus be eected. Accordingly, it is desired toprovide some means for reducing the sensitivity of the echo suppressorwhen noise currents flow in the four-wire circuit, without, however,rendering the suppressor insuihciently sensitive to voice currents. Viththe applicants arrangement, the apparatus for suppressing echoes isnormally in the condition of maximum sensitivity. The bias on the gridof the detector tube D1, for instance, is of normal value since thecondenser C1 is in a completely discharged condition, a circuit beingclosed through the armature of relays R2 and R2 and resistance r3.Accordingly, it voi-ce waves pass over line L1, for instance, the relayRl will operate in the normal way to cut off the echoes inline L2. Itshould be borne in mind that vrelay R1 is slightly less sensitive thanrelay R3 and considerably less sensitive than relay R2. Accordingly,normal voice waves in line Ll will, through the detector D1, operate notonly relay R1 but also relay R2 and relay R3. Since there is no chargeAon condenser C1, this operation of relay R2 is immaterial. Likewise fornormal amplitude of voice waves, the operation of relay R3 will not bevsufficiently long to cause the operation of relay R4 which is slowoperating. Thus it is seen that the bias on the detector tube D1 remainssubstantially normal during ordinary speech.

Occasional peaks of speech, however, may cause the ow of some gridcurrent in detector D1, and condenser C1 will be charged so as toincrease momentarily the negative bias on the detector tube. The result,of course, is a slight reduction of the sensitivity of the echosuppressor. This slight reduction is, for practical purposes,unobjectionable, since it is caused only by speech suficient-ly loud toinsure full operation of the echo suppressor apparatus. The irregii'-lar sensitivity reduction effect, however, can be minimized if theresistance r1 in the grid circuit of detector D1 is made sufficientlylarge. The effect of making this resistance very large is that any gridcurrent resulting from the speech peaks will' be so small thatpractically no charging of condenser C1 will be caused, it being bornein mind that the capacity of this condenser is quite high.

If, now, noise currents are present in line L,L of suiiicient amplitudeto cause false operation of relay R1 and the relays controlled thereby,relay R2 will be operated and will break the short-circuit on condenserC1. Likewise relay R3 will operate. Il the noise is oiY a fairlysteadynature and holds relay R3 in operated condition JforV asrniciently long period, the slow operating relay R.i will be operatedand a circuit will be closed through condenser C1, a battery, resistancer2, and inductance Il. The result will be additional negative grid biason the detector tube D1. This additional negative bias causes, ofcourse, a reduction of the sensitivity of the echo suppressor by raisingthe threshold 0In operation of the detector D., and this thresholdincrease will obtain as lon@ as the plate current due-to the steadynoise is suflicient to operate relay R3. Vhen the plate current in thedetector is reduced belowl the point at which relay R3 o ierates,the-relay of course releases and the c. arging of condenser C1 isinterrupted. Since relay. R2, however, is more sensitive than relay R3,the noise continues to thus the condenser C1 cannot discharge throughthe armature of relay R2. Since the capacity of this condenser is large,the loss of the condenser charge will be very slow and accordingly thethreshold of operation of the detector tube D1 will remain practicallyconstant. There may, however, be slight readjustments from time to time,as will be readily understood, osetting any condenser leakage Ythat mayoccur.

If, now, the noise inline L1 ceases or decreases Greatly, relay R2 willrelease andcondenser '1 will be discharged through resistance @thisdischarge being somewhat slow. If there is no return to great 'amplitudeof noise current, the discharge of the condenser will be completed andthe system will be returned to the normal condition. That is, thecondenser discharge will continue until the discharge circuit ijs again'broken by the operation 'of relay R2. v

It is apparent that the proper reduction of echo-suppressor sensitivityis accomplished in response to noise Aon either side of the circuit. `Asindicated above, it will be possible to connect the 'amplifier-detectorA2-D2 and the condenser C2 with relays R2 and R3 through eXtra windingsof those relays, but this introduces the disadvantage that, if noiseoccurs on both sides 'of the circuit there is excessive reduction 'ofsensitivity 'since the plate currents of the two detectors l)1 and holdit operated andV D2 would be added in the relays R2 and R3. Accordinglyit is highlydesirable to use separate relays, as shown in the drawing.It will be readily understood, of course, that, in response to noisecurrents in line L2 the apparatus shown in the lower half of the drawingoperates to charge condenser C2 through inductance I2, resistance r3,and the armatures of rela s R2 and Rz, causing adl threshold of ditionalbias on etector D2, and thatk when the noise decreases to the point ofrelease of relay R2, the condenser will be discharged slowly and thedetector bias removed.

It is to be noted that in the applicants echo suppressor, reduction ofsensitivity is obtained which is just sufficient to revent the.operation by noise currents of t e echo suppressing relays, no matterwhat may be the load characteristic of the detector tubes; thus batteryvariations etc., have no elfect on the device. It is to be noted furtherthat a margin of safety is provided through the adjustment of relay R3to be slightly more sensitive than relay R1, for instance, whichcontrols the suppressor relays proper.

Interaction between the two sides of the echo suppressor is preventedthrough the filtering action of condensers C1 and C2 and inductances I1and I2. v

`While the invention has been disclosed in one specific embodiment whichis deemed desirable, it is to be understood that this description is forthe purpose of illustration and does not limit the' scope of theinvention, which is to be determined from the appended claims. Y

What is claimed is:

l. `In a system for suppressing echoes in a two-way circuit, a detectorassociated with each side of the circuit vand responsive to Wavestherein, means controlled by each detector for blocking the oppositeside of the circuit, and means associated with each detector andappreciably responsive only to sustained noise in the circuit forraising the operation of the detector.`

2. Ina system for suppressing echoes in a two-way circuit, a detectorassociated with 1 the circuit and responsive to' each side of Wavestherein, means controlled by each detector for blocking the oppositeside of the circuit, and means associated with each detector andappreciably responsive only to sustained noise in the circuit forraising the threshold of operation of the detector toV a degree justsuilicient to prevent operation by the noise. v

3. In a system for suppressing echoes in a two-way circuit, adetectorassociated with each side of the circuit and responsive to wavestherein, means controlled by each detector for blocking the oppositeside of the circuit, means associated with each detector and appreciablyresponsive only to sustained noise in the circuit for raising thethreshold of o degree just su the noise, and `means associated with eachdetector for lowering its threshold of op-v eration in response todecrease of the'noise.

without biasing charge thereon, means for preventing the charging ofsaid condenser ingresponsejto voice waves or'normal noise in thecircuit, means associated with each detector and responsive to sustainednoise in the circuitfor charging the condenser to place a negativevoltage on the detector grid, and means'associated with each deerationof the detector lto a4 cient to prevent operation .byv

tector for discharging the condenser inV response to a decrease of thenoise.

5. In a system for suppressing echoes in a two-way circuit, athree-electrode vacuum tube detector associated with each side of thecircuit and responsive to waves therein, means controlled by eachdetector for blocking the opposite side ofthe circuit, a condenser inthe grid-filament circuit of each detector,said condenser being normallywithout biasing charge thereon, means for preventing the charging ofsaid condenser in response to voice waves or vnormal noise in thecircuit, meansl associated with each detector and responsive tosustained noise in the circuit of sulicient strength to operate saidcontrolled means for charging negative voltage.

the condenser to place a on the detector grid, and means associated witheach detector fordischarging the condenser in response to a decrease ofthe noise.

In testimony whereof, I 'have signed my name to this-'specification this18th day of February, 1929.

Y DOREN MITCHELL.

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